Blodthirst does this:

1: Makes you attack EVERYTHING that you can see when you enter a room.
2: Gives you a boost to hit/dam/
3: Gives you haste
4: Removes physical maladictions placed before the bloodthirst. (They will return when the thirst ends?)
5: Allows you to spellbane area attacks like a scout.
6: Makes you less vulnerable to lag.
7: Makes you AND others that look at you how healthy you are (And how much moves/mana you have.) Only those fighting you can see if you convulse or not etc.
8: Once the bloodthirst ends, you are left slowed and drained from both mana and moves, (all maladictions return to you.?)


So, to sum it up... The bad things are:
You attack everything, so anyone with summon can make life a living hell for you when you thirst. You attack possible allies, if you thirst in a crowded area, you'll have a hard time retreating since you'l bounce around on everything when you flee.
You can't see your HPs and EVERY attack against you just says scratches, so you'll have no clue about how hurt you are.
When the thirst ends, and you don't EVER know when, you will be very weakened and vulnerable.
And again... You will have a hard time to escape in crowded areas.

Basically, for most people that ain't good with playing thirsting ragers, bloodthirst is a desperate suicide move. Against a smart player, outside the village, bloodthirst will be a death sentance, unless you are really good at using it.